1. Field of the Invention
The present invention relates to microwave band-pass filters, more particularly to filters made of plane E waveguide technology with a printed dielectric insert, the filter being suitable for insertion in a transmission subsystem produced on printed circuit. It applies more particularly to wireless telecommunication systems operating in the millimetric domain and needing to satisfy high spectral purity requirements.
2. Background of the Invention
In the context of wideband bidirectional communications using a geostationary satellite in the Ka band, it is necessary to use, in the terminals intended for the consumer market, an output filter for attenuating the spurious signals located outside the useful band, typically 29.5-30 GHz. This filter must more specifically reject the frequency of the local oscillator, located typically at 28.5 GHz. To satisfy the consumer market requirements, this filter must be inexpensive.
Given the requirements, the use for this of a waveguide type technology according to various methods is known, in particular:
single or multi-mode cavity filters coupled between themselves by inductive or capacitive irises;
evanescent mode filters;
E plane type filters, with metallic inserts or printed dielectric inserts, commonly called FINLINE.
The basic technology used in the present invention relates to the last of the above and is illustrated in FIG. 1.
In FIG. 1, a microwave waveguide 101 of rectangular section is divided into two identical parts by a plane dielectric substrate 102 situated in the E propagation plane of this guide. This substrate offers low losses and is of minimal thickness (less than 0.2 mm for example) so as not to degrade the quality factor of the guide. However, in this figure, and in the other figures, the thickness of the substrate has been shown greatly enlarged for improved legibility.
The substrate 102 comprises on at least one of its sides printed conductors 103 electrically linked to the internal surfaces of the guide which support the substrate 102 and the topology of which determines the response required for the filter. For simplicity, the term “conductive inserts” will be used to describe these conductors electrically linked to the guide.
The main benefit of this technology is that it can be incorporated and interfaced easily with other planar technologies, such as the microstrip or suspended microstrip technologies. This then means that the filtering function can be incorporated in printed circuits on the main board of the transmission system.
The band-pass filter topology most commonly used in the technologies represented in FIG. 1 consists in using n+1 inductive inserts grounded by being electrically linked to the internal surfaces of the guide, where n is the order of the filter. These inserts are spaced at intervals approximately equal to half the guided wavelength and are theoretically printed on a single side of the substrate. However, to minimize the sensitivity of the response of the filter to production tolerances, the inserts are preferably printed roughly identically on both sides of the substrate, but they are still connected to the internal walls of the guide. The response curve for the band-pass filters obtained in this way is of the Chebyshev type.
To obtain the necessary spectral selectivity, a high order filter can theoretically be used. The filter then obtained has large physical dimensions and is highly sensitive to production errors relating to its dimensions. It is therefore in practice very difficult, even impossible, to produce.
The present invention proposes a new microwave band-pass filter structure which can be used in particular to remedy the dimensioning problems while maintaining the high performance levels and low production costs.